The present application concerns the field of electromagnetic based detection and sensing, in particular in an industrial environment, and relates particularly to an inductive proximity sensor, detector or switch, which can work in a magnetic field and can detect both ferrous and non-ferrous metals.
More specifically, the present application concerns in particular an inductive sensor or detector of the type comprising:                an inductive coil defining a front working plane of the sensor and associated with a covering plate or a plane part of a housing, said plate or part being disposed substantially perpendicular to the coil axis and parallel to its front working plane,        means for supplying the coil or inductance repeatedly with current,        means for processing signals which correspond to the voltages induced in said coil or inductance when fed, said induced voltages being influenced by the presence of objects or articles (targets) situated within a given detection area of said sensor.        
Inductive proximity sensors using a coil as the sensitive element are already known. The working principals of this type of sensors are as follows.
When the coil with its associated flux field is placed close to the conductive target, the field establishes electric currents in the target. These currents are eddy currents, i.e. closed loops of induced current circulating (in a direction opposite to the current in the coil) in planes perpendicular to the magnetic flux, and generating their own magnetic field. Eddy currents normally run parallel to the coil windings and to the target surface. The eddy current flow is limited to the area in the target within the inducing magnetic field (see FIG. 1).
The magnetic flux associated with the eddy currents opposes the coil's own magnetic flux. Decreasing the target-to-coil gap changes the inductance of the coil and thus the net flux of the system. The result is a change in the impedance of the coil and a voltage change across the coil. It is this interaction between the coil and the eddy current fields that is the  basis for determining target-to-coil position information with an eddy current position sensor.
The most common way of converting the impedance of the coil into electrical signal parameters is to make an LC generator with the inductance coil L as its sensing element. As the impedance of the inductance coil changes, parameters of the periodic signal at the output of the generator, such as amplitude and frequency, also change; thus making it possible, by providing a suitable electronic circuit, to detect a target as it approaches the sensing element of a detector. Similar designs were used in detectors described in the following patent and patent application documents: U.S. Pat. Nos. 4,942,372, 6,215,365, 6,664,781, DE-A-40 31 252, EP-A-0 304 272, 5,504,425, 6,335,619, 5,519,317, 5,952,822, EP-A-0 403 733, WO-A-00/76070.
As closest prior art, U.S. Pat. No. 5,027,066 discloses a distance detecting circuit that generates an electrical signal proportional to the linear displacement of an object. The functional diagram of the concerned device is shown in FIG. 4 of said document and the concerned detector is actually a generator built around an operational amplifier 67. Resistors R4 and R5 are used to set the required gain of the amplifier. The output of the amplifier is connected via resistor R6 to an oscillating LC-circuit (elements 71 and 65). Coil 65 is the primary winding of the transformer and its secondary windings 63 and 64 are connected to said primary winding via a moving core 18. The linear movement of the core 18 changes the amount of induction factor between the transformer windings. Correspondingly, signal parameters change at the input of the detecting device 24 and the latter generates a voltage at its output that is proportional to the linear displacement of the core linked mechanically to the object.
The present application proposes an inductive position detector or sensor which shows at least some of the following improvements and additional features in comparison with the detectors known from the aforementioned documents, in particular from U.S. Pat. No. 5,027,066:
1. The detector or sensor should detect the presence of a target located at a certain distance in front of it whether this target is made of any ferromagnetic metal or of any non-ferrous/non-ferromagnetic metal.
2. The detector or sensor should be able to differentiate these two types of targets (ferromagnetic/non-ferrous). 
3. The detector or sensor should remain operational when exposed to the effect of a constant or alternating magnetic field of industrial frequency.
4. The design of the detector or sensor should allow for its flush mounting with the frame of any material.
5. The sensor should be able to detect and evaluate an approaching article or object without any physical connection with the latter.
It is an aim of the present application to propose an inductive proximity (presence or position) sensor or detector showing at least some of the aforementioned benefits or improvements.